Laser or ozone generator in which a broad electron beam with a sustainer field produce a large area, uniform discharge

ABSTRACT

Apparatus for and a method of producing controlled discharges substantially throughout a large volume of a gaseous medium by generating in an enclosure a controlled density of free electrons in the medium and controlling the electron temperature of the free electrons to a level preventing a substantial increase in their density by a self-regenerative ionization process so that for a wide range of uniformity of both the density and temperature of the medium, a stable and controlled discharge is produced in the medium suitable for the intended use of the medium. Apparatus for and the method of producing a discharge in accordance with the invention is useful for the production of lasting action, electrically conductive ionized gas for use in magnetohydrodynamic (MHD) devices and the like, or to produce or facilitate carrying out chemical processes such as, for example, the generation of ozone and the like.

United States Patent 11 1 3,702,973

Daugherty et al. Nov. 14, 1972 [54] LASER OR OZONE GENERATOR IN3,576,583 4/1971 Uno ..313/74 WHICH A BROAD ELECTRON BEAM 3,641,4542/1972 Krawetz ..33 1/945 WITH A SUSTAINER FIELD PRODUCE A LARGE AREA,UNIFORM OTHER PUBLICATIONS DISCHARGE Dumanchin et al.: Comptes Rendus,vol. 269,

November, 1969, pp. 916- 917.

[72] Inventors: Jac g y, winChester; Beaulieu: DREV Memorandum M-2005/7OJanuary,

Diarmaid H. Douglas-Hamilton, 1970 Boston; Richard M. Patrick,

Winch er; n g g- Primary Examiner-Ronald L. Wibert 311 Of Mass-Assistant ExaminerEdward S. Bauer [73] Assigneez Avco Corporation,Cincinnati Ohio AtzorneyCharles M. Hogan Melvin E. Frederick [22] Filed:Sept. 17, 1970 [57] ABSTRACT [21] Appl. No.: 72,982 Apparatus for and amethod of producing controlled discharges substantially throughout alarge volume of 521 US. Cl ..331/945 PE, 204/176 204/313 a gasews mediumby generating in enclmre controlled density of free electrons in themedium and controlling the electron temperature of the free electrons toa level reventing a substantial increase in 51 111 11. (:1 ..H0ls 3/00their density by apselfqegenerative ionization prosess Field Of Search 31 3 1 l 1; so that for a range f uniformity of both the 204/ 316; 310/11 sity and temperature of the medium, a stable and controlled dischargeis produced in the medium suitable 204/3l6, 310/11, 313/74, 3l5/lll,331/945 G References Cited for the intended use of the medium.

UNITED STATES PATENTS Apparatus for and the method of producing a2,030,492 2/1936 Applebaum ..313/74 X Charge n accordance w1th the1nvent1on 1s useful for the production of lasting action, electricallycon- 2,686,275 8/1954 Cohen ..313/74 (motive ionized as for use in maetch dmd namic 3,403,353 9/1968 Lamb, Jr. et al ..331/945 (MHD) devicesgand the like, f to g or 3,555,451 1/1971 W1tte etal. ..331/94.5facilitae carrying out chemic a1 processes Such as, for 2,333,842 11/1943 Casc1o et al. ..313/94 x example, the generation of ozone and thelike. 2,373,661 4/1945 DePhillips ..313/74 2,429,217 10/1947 Brasch..313/74 x 29 Claims, 6 Drawing Figures TO VACUUM PUMP GAS FLOW T0FILAMENT POWER SUPPLY 30 T0 E SUSTAINER V cmcun 53 commuous R PULSED FLOW PATENTEDnnv 14 1972 3,702 973 SHEET 2 or 3 LASER OUTPUT commuous GASOR FLOW "PuLsED FLOW I 63 LASER OUTPUT 5 mg E JACK 0. DAUGHER TYDIARMAID DOUGLAS-HAMILTON PM? RICHARD M. PATRICK SUS'ITEINER 5| EVANCIRCUIT INVENTORS 4&4 s3 i 5 ATTORNEYS sum 3 or 3 T N E M A H F 0 TPOWER SU PPLY JACK D. DAUGHERTY' DIARMAID DOUGLAS- HAMILTON RICHARD M.PATRICK EVAN R. PUGH LASER OUTPUT a F F F 5 F GAS FLOW ATTORNEYS LASEROR OZONE GENERATOR IN WHICH A BROAD ELECTRON BEAM WITH A SUSTAINER FIELDPRODUCE A LARGE AREA, UNIFORM DISCHARGE The present invention in itsbroadest sense is directed to the production of and apparatus forproviding useful controlled discharges in a gas at pressure levels andvolumes such that discharge stabilization by electron pair diffusion toconfining walls is negligible, that is, the discharge is not walldominated.

In one embodiment, the invention may comprise means for increasing ifnot providing the desired electrical conductivity of the gaseous workingmedium in MHD devicessuch as generators and accelerators. It is equallyapplicable to other devices and the like that require or useelectrically conductive or ionized gas.

In another embodiment, the invention comprises means for producing ozonewherein the working medium may comprise oxygen or air which is passedthrough a discharge comprising an independent source of electrons and anelectric field in accordance with the invention. Since the electricfield is decoupled from the production of electrons optimum conditionsfor ozone formation are attainable without severe requirements onballasting as present in the use of a Townsend discharge, or onelectrode geometry as present in the use of corona discharge. Becauseuniform conditions are provided in the positive column, the overallenergy efficiency is increased and heat dissipation involved in theprocess is reduced. In a still further and preferred embodiment, theinvention comprises a high-power gas laser which is volumetric incharacter and that can be sealed in all three characteristic dimensionsas well as in pressure level. A controlled discharge is created whereelectron-ion diffusion to the walls is negligible. 2

While the preferred embodiment of the present invention will bedescribed in connection with an electrically excited nitrogen (N2),carbon dioxide (CO and helium (He) laser, it may, as noted above by wayof example, be applied to other systems where a conducting ionized gasis required or useful and including, but not restricted to, gasconstituents other than N,, CO, and He as well as other lasing systems.Since the discharge produced by this invention does not requireionization by the discharge electrons, in a lasing environment, adischarge in accordance with the invention can be adjusted to thecorrect electron temperature for most efficient laser operation.Moreover, a laser in accordance with the invention is volumetric in thesense that the proper gas temperature and lower laser stateconcentrations are maintained by means other than diffusion through thegas to cooled side walls. Further, apparatus in accordance with theinvention may be operated in the flowing gas as well as the static pulsemode.

Light amplification by stimulated emission of radiation (laser) hasextended the range of controlled electromagnetic radiation to theinfrared and visible light spectrum. A laser produces a beam of coherentelectromagnetic radiation having a particular well-defined frequency inthat region of the spectrum broadly described as optical. This rangeincludes the near ultraviolet, the visible and the infrared. Thecoherence of the beam is particularly important because it is thatproperty which distinguishes laser radiation from ordinary opticalbeams. On account of its coherence, a

laser beam has remarkable properties which set it apart from ordinarylight which is incoherent. While the maser (microwave amplification bystimulated emission of radiation) and the laser are based on the sameprinciples of statistical and quantum mechanics, the problems and thephysical embodiments for achieving laser action are completely differentfrom those for masers.

Coherence, the essential property of lasers is of two kinds: spatial andtemporal. A wave is spatially coherent over a time interval if thereexists a surface over which the phase of the wave is the same (or iscorrelated) at all points. A wave is time-coherent at an infinitesimalarea on a receiving surface if there exists a periodic relationshipbetween its amplitude at any one instant and its amplitude at laterinstants of time. Perfect time coherence is an ideal since it impliesperfect monochromaticity, something which is forbidden by theuncertainty principle.

Laser beams have a number of remarkable properties. Because of theirspatial coherence, they generally have an extremely small divergence andare therefore highly directional. For example, a ruby laser beam oneinch in diameter at the source will be about four feet across on asurface ten miles away. The very best that could be accomplished overthe same distance with an incoherent source, such as an arc lamp at thefocus of a six-foot parabolic mirror, would be a beam spread over anarea more than one-third of a mile across. Another important feature oflasers is the enormous power that can be generated in a very narrow wavelength range. Under certain operating conditions, nearly monochromaticbursts of millions of watts can be produced. To get comparable radiationintensity from a black body, it would have to be raised to a temperatureof hundreds of millions of degrees-a condition not practicallyachievable. A laser beam, because it possesses space coherence can befocused to form a spot whose diameter is of the order of one wave lengthof the laser light itself. Enormous power densities are thus attainable.For example, the focused output of a 50-kilowatt infrared burst from alaser can have a radiant power density of the order of 10 watts/cm; thisis about million times the power density at the surface of the sun.Extraordinarily high temperatures, orders of magnitude greater than thatat the sun, can be generated at the small area which absorbs thisconcentrated radiation. Furthermore, since the electric field strengthof an electromagnetic wave is proportional to the square root of itsintensity, the field at the focus of the laser beam can be millions ofvolts per centimeter. A promising potential of lasers comes from timecoherence. It is this property which permitted prior art exploitation ofradio and microwaves for communications. However, laser frequencies aremillions of times higher than radio frequencies, and hence are capableof carrying up to millions of times more information. In fact, onesingle laser beam has in principle more information-carrying capacitythan all the combined radio and microwave frequencies in use at thepresent time.

Accordingly, systems applications of lasers are useful for communicationin space, on earth, and under sea. Military surveillance and weaponssystems, mapping, medical, mining, manufacturing, and computertechnology may also include lasers.

Two conditions must be fullfilled in order to bring about laser action:(1) population inversion must be achieved and (2) an avalanche processof photon amplification must be established in a suitable cavity suchas, for example, an optical cavity. Population inversion can, forexample, be accomplished if (I) the atomic system has at least threelevels (one ground and at least two excited levels) which can beinvolved in the excitation and emission processes and (2) the lifetimeof one of the most energetic of the excited states is much longer thanthat of the other or others.

When a system is in a condition where light (photon) amplification ispossible, laser action can be achieved by providing l means forstimulating photon emission from the long-lived state, and (2) means forcausing photon amplification to build up to extremely high values. Inthe usual embodiment, this is accomplished by fashioning the mediumcontaining the active atoms into a cylinder with perfectly (as far aspossible) parallel ends polished so highly that the surface roughness ismeasured in terms of the wave length of the laser. The ends may besimply polished metal or they may be silvered or dielectric coated sothat they behave as mirrors which reflect photons coming toward themfrom the interior of the cylinder. Such a structure, whether the mirrorsare within or outside the container, is called an optical cavity. If nowpumping means, such as for example, an electric discharge acts on themedium and brings about population inversion of the long-lived statewith respect to another lower energy excited state even though thelong-lived state is only relatively longlived, in a small fraction of asecond there will be spontaneous emission of photons. Most of thesephotons will be lost to the medium but some of them will travelperpendicular to the ends and be reflected back and forth many times bythe mirrors. As these photons traverse the active medium, they stimulateemission of photons from all atoms in the long-lived state which theyencounter. In this way the degree of light amplification in the mediumincreases extraordinarily and because the photons produced by stimulatedemission have the same direction and phase as those which stimulate themand assuming the optical quality of the laser media is suitable, theelectromagnetic radiation field inside the cylinder or cavity iscoherent. In order to extract a useful beam of this coherent light fromthe cavity, one (or both) of the mirrors is made slightly transmissive.A portion of the highly intense beam leaks through the mirror, andemerges with regularly spaced wave fronts. This is the laser beam.

Parallelism of the mirrors is a rigorous geometrical requirement in lowgain lasers. Thus, in low gain lasers, if the mirrors are not preciselyparallel, the light rays that build up in the cavity will tend todigress further and further toward the edges of the mirrors as they arereflected back and forth between the mirrors, and finally they will bedirected out of the cavity altogether. It is essential that anydeviation from parallelism be so small that the coherent photon streamswill reflect back and forth a very large number of times to build up therequired intensity for laser action.

In all diffraction limited optical configurations such as thosediscussed above, coherent wave fronts appear to originate from a commoncenter and so they can, by use of a lens, be made plane-parallel andhence; except for diffraction effects, non-divergent. In high gainlasers other optical configurations such as oscillator amplifierconfigurations and unstable resonators are used. A characteristic ofthese devices is that the photons only make a small number of passesthrough the laser medium. In present operational lasers, the photon isreflected about only two or three times.

By way of example, a continuously operating gas laser is disclosed in anarticle, Population Inversion and Continuous Optical Maser Oscillationin a Gas Discharge Containing I-Ie-Ne Mixture, Physical Review Letter,6, page 106, 1961. In the usual embodiment of static gas, prior art gaslasers, the gas is statically contained in a tube about centimeterslong. The mirrors which form the ends of the optical cavity are disposedeither inside the tube or external to it. Pumping is accomplished inthis system by electrical excitation (either radio frequency or directcurrent).

In addition to the helium-neon gas laser system, other gas laser systemshave been achieved with helium, neon, argon, krypton, xenon, oxygen, andcesium (the last optically pumped in the gaseous state) as emittingatoms.

Other systems include carbon dioxide, helium, and nitrogen. For a morecomplete discussion of the highpower flowing system including carbondioxide, helium and nitrogen reference is made to patent application ofC. K. N. Patel, Ser. No. 495,844, filed Oct. 14, 1965 abandoned in favorof continuation-impart application, Ser. No. 814,510 filed Mar. 28,1969, now US. Pat. No. 3,596,202, and assigned to Bell TelephoneLaboratories, Inc. Such a high-power laser typically includes tworeflectors forming a suitable resonator or cavity, a tube forming theside walls of the laser, suitable pumping apparatus including a cathode,anode and directcurrent sources connected in appropriate polaritybetween the anode and the cathode; inlet apparatus; a source of carbondioxide, helium, and nitrogen connected to the inlet apparatus; andequipment for exhausting the spent gases from the laser or for coolingand separating them for reuse.

As 'indicated hereinabove, a laser output may be generated in variousmedia (i.e., crystals, semiconductors and gases) by pumping orintroducing energy to create an inversion where a large number of theatoms are in high energy levels to support photon emission. In prior artgas lasers, whether flowing or static, the lasers were pumped or excitedby using a diffusion controlled electrical discharge in a small tubemaintained at a low pressure. Typically, in such gas discharge tubes(typically of the order of l centimeter in diameter) operating at lowpressures (about l-IO torr) there is a loss of electron-ion pairs fromthe center of the plasma to the side walls of the tube by radialdiflusion (so-called ambipolar diffusion of ion-electron pairs). For asteady state operation of the discharge, this loss must be made up by anet ionization rate in the plasma which exactly balances the diffusionloss rate. This required ionization rate dictates what temperature theelectrons must have to sustain the discharge, and hence what applied E/Nis needed to give the electrons that temperature. For long tubes E/N isdefined by the applied voltage divided by the tube length and gasdensity.

In such situations, the discharge can be said to be ballasted by thetube walls, i.e., since radial diffusion of the electron-ion pairs isfast, any small local increase in electron density is reduced bydiffusion. This fact makes such discharge radially and axially uniformas well as quite reliable and simple to produce.

The plasma (neutral gas plus electron-ion pairs) contained inside theelectric discharge tube tends to remain radially smooth as long as thetime required for the electron-ion pairs to diffuse to the surroundingwalls is equal to the ionization time such as, for example, the timerequired to double the electron density.

Since the ambipolar diffusion time is generally proportional to theproduct of the gas pressure and the tube diameter squared for largediameters, this ambipolar diffusion time can, under some circumstances,become long compared to the ionization time in the tube, especially forhigh ionization rates, large diameter tubes and high pressures. In thislatter situation, the discharge is no longer ballasted by the presenceof the tube walls, i.e., local increases in the electron density are notimmediately diffused to the walls where they are reduced by wallrecombination, etc. Accordingly, local nonuniformities can be producedby these higher electron densities and the fast-growing non-uniformitiescan become worse. Often the result is that the previously uniform glowdischarge turns into arcs, streamers or current spokes. This lattercondition often is a plasma that is very inefficient, and often uselessfor certain purposes.

From the above, it will be seen that in high-pressure, large diameterdischarge tubes the tendency is for any local increase in electrondensity not to be damped by diffusion to the confining walls. Uponoccurrence of such disturbances, one can reduce their tendency to growby reducing the ionization rate which means a lower electron temperaturesince the local ionization rate is a function of the local electrontemperature. A lower electron temperature, however, requires that alower electric field must be applied. The proper balance is a criticalone: too high an electric field can allow the high pressure largediameter discharge to spoke, but if too low an electric field isapplied, the discharge cannot be started in the first place. Further, athigh pressures, it is generally found that an applied voltage orelectric field large enough to start a discharge is also large enough tocause the discharge to be radially non-uniform and, for example, spoke.For the preceding reasons, it will be seen that if a discharge tube orcavity has a sufficiently large volume, maintenance of a controlleddischarge therein by diffusion to the walls is not possible. As usedherein the term controlled discharge means in a gaseous medium adischarge having predetermined properties which although such propertiesmay vary in space and time, they remain at-least within desired limitsfor the time that the discharge exists. Such properties include but arenot limited to the electronic and molecular states of the gaseous mediumas well as the optical, electrical and chemical qualities of the medium,and its heating, ionization, dissociation, and recombination rates. Acontrolled discharge provided in accordance with the invention has acharacteristic time which is substantially the duration of the time thatsustainer current flows in the gaseous medium as a result of the motionof secondaryelectrons generated in the gaseous medium under theinfluence of an electric field termed herein a sustainer electric fieldmore fully described hereinafter. For the case of a flowing mediumwherein flow time through the cavity or working region is less than theduration of sustainer current in the gaseous medium, the characteristictime is the flow time through the cavity.

This invention is an improvement over that disclosed in pending patentapplication Ser. No. 859,424 filed Sept. 19, 1969 by James P. Reilly,abandoned in favor of continuation-in-part application Ser. No. 50,933,filed June 29, 1970, to which reference is made, and assigned to thesame Assignee as this patent application.

It is an object of the invention to provide apparatus for and a methodof producing controlled discharges in a gaseous medium.

It is another object of the invention to provide a controlled dischargein a gaseous medium in a controlled 7 manner with predetermined effecton background temperature, density and pressure of the medium.

A still further object of the invention is to provide apparatus for anda method of producing controlled, large, volumetric discharges withoutthe inherent ionization instability that occurs when the dischargecurrent itself produces the ionization.

A further object of the invention is to provide apparatus for and amethod of producing spatially uniform discharges in a gaseous mediumthat can be used, for example, to provide a lasing medium chemicalreaction processes, mediums for Ml-lD devices and the like and otherapplications where a conducting gaseous medium is necessary or useful toachieve a desired result.

Another object of the present invention is to provide apparatus for anda method of producing a population inversion suitable for use in a gaslaser oscillator or amplifier.

It is another object of the present invention to provide apparatus forand a method of producing laser action in a flowing gas by electricalexcitation.

A still further object of the invention is to provide a method of andapparatus for controlling the gas temperature in a gaseous laser byproper choice of gas flow velocity and input power to increase theefficiency of the lasing of the gaseous laser.

A still further object of the present invention is to provide a methodof and apparatus for producing laser action in a flowing gas bygenerating free electrons, and an electrical discharge to maintain theoptimum electron environment to produce the lasing action.

A still further object of the present invention is to provide a methodof and apparatus for producing laser action in a flowing gas byelectrical excitation comprising an electron beam to create electronsand a DC voltage to produce a discharge which maintains the optimumelectron environment to produce lasing action.

Due to the ability to control the distribution of a discharge in alasing medium, a still further object of the invention is to permit inan electrically excited gas laser an arrangement of electricalexcitation means resulting in optimum optical qualities.

The novel features that are considered characteristic of the inventionare set forth in the appended claims. The invention itself, however,both as to its organization and method of operation, together withadditional objects and advantages thereof, will best be understood fromthe following description when read in conjunction with the accompanyingdrawings in which:

FIG. 1 is a perspective view with parts broken away of the apparatus inaccordance with the invention;

FIG. 2 is a sectional view taken on lines 2-2 of the apparatus shown inFIG. 1;

FIG. 3 is a sectional end view taken on lines 33 of the apparatus shownin FIG. 2;

FIG. 4 is a perspective view with parts broken away showing details ofthe electron source;

FIG. 5 is a perspective diagrammatic view illustrating method ofoperation and coordinates associated with electron generation, gas flow,and lasing activity; and

FIG. 6 is a block diagram of the circuitry associated with the electrongun and sustainer electrodes.

Attention is now directed to FIGS. 1-6 which show various details oflaser apparatus incorporating the invention. Apparatus is shown in thesefigures wherein a gaseous medium capable of producing lasing action suchas, for example, a mixture comprising 16% CO 34%N and 50% He is suppliedfrom a suitable conventional source (which may comprise a plenum chamberand diffuser not shown) to suitable means defining a cavity or workingregion 10 of laser apparatus 12 via gas inlet means 11. The cavity orworking region 10 of the laser apparatus (generally designated by thenumeral 12) is shown by way of example as being generally rectangular inconfiguration. The term cavity as used herein means not only one that isdefined by walls, but also one that is not defined by walls or the likesince in certain cases such are not essential to carrying and/or usingthe invention. As best shown in FIGS. 2 and 3, the rectangular workingregion 10 comprises oppositely disposed top and bottom walls 14 and 15adapted to receive and support respectively mirror holder and adjustmentassemblies 21 and 22 more fully described hereinafter. Carried on theinner surfaces, the top and bottom walls 14 and 15 are oppositelydisposed arcuate flow members 16 and 17 which are arranged and adaptedto function to provide a smooth laminar flow through the working region10. The mirror assemblies 21 and 22 are each disposed in members 16 and17 and recessed to provide minimum disruption of flow and minimizespurious arcing. .Oppositely disposed side walls 18 and 19 are sealablyattached to the top and bottom walls 14 and 15, side wall 18 beingprovided with a circular opening to sealably receive electron gunapparatus 25 more fully shown and described hereinafter. oppositelydisposed to the aforementioned circular opening in side wall 18 is arecess in side wall 19 for receiving a flush mounted electricallyconductive electrode plate 52 more fully described hereinafter. Theaforementioned components other than electrode 52 defining the workingregion 10 are preferably comprised of an electrical nonconductivematerial, such as, for example, Lucite, Melamine, Fiberglass Epoxy, andthe like. 3

As shown in FIGS. 1 and 4, the electron gun generally designated by thenumeral 25 includes a rectangular electron source comprising anelectrically conductive enclosure 26 constructed of stainless steel orthe like and open at one end. Within enclosure 26, electrons aregenerated in conventional manner by thermionic emission from a pluralityof spaced filaments 27 which are supported within and near the rearportion of enclosure 26 by a plate 28 comprised of electricallynonconductive material. Filaments 27 are supported by electricallyconductive stand-offs 29 which are coupled to a source of filamentcurrent 30. The filaments are heated in conventional manner by source 30to produce the thermionic emission. The enclosure 26 is mechanicallysupported within and insulated from the outer cylindrical wall 36 bysupports 33 and 34 which also provide electrical connection to the pulsecircuit '40. Supports 33 and 34 permit application to enclosure 26 thepotential necessary to control the amount of electrons generatedtherein. One form of control may be provided as shown via a reticulatedscreen grid 35 electrically and mechanically connected to the enclosure26 and covering the open end thereof.

A conventional pulse circuit 40 (see FIG. 6) coupled to grid 35 viasupports 33 and 34 and enclosure 26 provides the necessary potential tocontrol the amount of high energy electrons released by the electrongun. The pulse circuit 40 is triggered or actuated by a timing circuit41. Actuation and control of the electron gun is more fully describedhereinafter. Broadly, the electron emitter or gun provides an abundanceof high energy electrons which are defocused and directed toward theworking region 10 through the screen grid 35 (see FIG. 5).

The volume surrounding the electron gun within wall 36 is evacuated by avacuum pump (not shown) in conventional manner and the electron gunmaintained at a low pressure to provide an optimal environment for thefree electrons generated therein to pass unhampered through screen grid35 and be attracted and accelerated toward a reticulated electricalconducting plate 45. Plate 45, made of stainless steel or the like, ismaintained at a potential high compared to that of screen grid 35.Electrons generated at the filaments 27 are strongly accelerated towardplate 45 and a portion passthrough the plurality ofholes 46 provided inplate 45. A thin sheet of material or diaphragm 47 is disposed betweenthe working region 10 and the electron gun to permit the existence ofseparate pressure regimes. Diaphragm 47, which may be at least in partsupported by plate 45, must possess adequate structural stability towithstand any required pressure differential (the vacuum in the electrongun 25 and the pressurized gas flow in the working region 10) andcomposed of a material arranged and adapted to transmit the maximumnumber of electrons without absorbing an excessive portion of theirenergy which can reduce efficiency and/or result in failure of thediaphragm. While preferably composed of metal, the diaphragm 47 may becomposed of either nonconductive or conductive material.

After electrons from screen grid 35 pass first through the holes 46 inthe plate 45 and then the diaphragm 47, they enter the working region 10by passing through a reticulated cathode 50 which may be constructed ofa wire mesh and insulated, if desired, from .the electron gun 25 by aring of non-conducting material 51. In the working region 10, electronenergy is maintained by a sustainer electric field between oppositelydisposed anode plate 52 and previously mentioned cathode 50 which arecoupled to the sustainer circuit 53. Cathode 50 which may be comprisedof a wire mesh grid as previously noted prevents, for the electron beamand sustainer electric field arrangements as shown, damage to thediaphragm 47 from spurious arcs which may be otherwise inadvertentlystruck between the anode 52 and/or cathode 50 and diaphragm 47. A highvoltage direct current potential is typically maintained between anode52 and cathode 50 by a conventional sustainer circuit 53 which maycomprise capactive discharge means charged by power supply 54 andtriggered by timing circuit 41 for pulsed operation. The examplehereinbefore given is for a shower head type electron beamwhich covers abroad area, however, the same result may be accomplished by theprovision of a rapidly swept beam of electrons over a broad area.

The production of a volumetric controlled discharge, which for theembodiment illustrated in FIGS. 1-6 comprises the excitation andinversion of a gaseous medium in the working region between thesustainer anode 52 and cathode 50 is provided in accordance with theinvention in two steps. A discharge as used herein is, in an ionizedmedium, the flow of current under the influence of a sustainer electricfield or fields. While primarily described herein is the use of DCvoltages with inter-cavity electrodes, to provide a sustainer field theinvention described herein includes the use of radio frequencyelectromagnetic fields, inductive electrode structures, capactiveelectrode structures, movement of an electrically conductive medium inthe presence of an applied magnetic field, and the introduction of laserenergy into the working cavity to provide the sustaining electricfields. For a more complete discussion of the basic process hereinvolved, reference is made to the aforementioned patent applicationSer. No. 859,424 filed Sept. 19, 1969. The present invention comprisesan improvement over the aforementioned patent application in theprovision of ionizing radiation, such as, for example, the provision ofhighly efficient ionizing radiation through the use of electron gunmeans rather than high voltage discharge means or the like disclosed inthe aforementioned application. The ionizing radiation provided inaccordance with the invention provides a source of secondary electronsat very low temperatures and increased efficiencies heretoforeunobtainable since theory indicates that the only way comparableconditions in high power, high pressure devices can be duplicated is tooperate the pulser circuit of the aforementioned patent application atlevels of the order of one million volts and/or high repetition pulserates a result not easy if not impossible, of practical attainment.

As taught in the aforementioned patent application, a principal featurein providing a volumetrically scalable discharge is the control of gastemperature and discharge uniformity wherein an electrical discharge orthe like produces free electrons and ionization of the working medium ina sustainer electric field. Electron temperature, which is a function ofE/N in any gas mixture, is controlled by adjustment of the sustainerelectric field E and control of the gas density N. In flow applications,proper design determines the allowable temperature rises (A1) in the gasand the corresponding density (A N) in the gas. In pulsed applications,the heat capacity of the gas, the pulse width and the effect of pressurewaves must be considered in the proper control of A N. If the electrontemperature is kept sufficiently low so that the ionization due to thesustainer field is small compared to ionization due to theaforementioned free and preferably high energy electrons, the volumetricdischarge can be maintained in a controlled manner to high pressures.For example, controlled discharges in accordance with the invention ofup to one atmosphere have been established.

The aforementioned patent application disclosed in detail the provisionof a short, high voltage pulse substantially inductively spreadthroughout the volume of the working medium. This is accomplished by theprovision of a plurality of electrodes and a short pulse. The dischargeis uniformly provided or spread throughout the cavity containing theworking medium because the volumetric discharge through all of theelectrodes offers the least inductive impedance and thereby makes thecurrent in the short pulse flow reasonably uniform throughout the volumeof the cavity containing the working medium. An important criterion forthis arrangement to be practically effective is that the inductiveimpedance of the short pulse discharge circuit be comparable to theresistive impedance of the discharge. This has been accomplished inaccordance with the aforementioned patent application and produceduniform ionizations over large volumes in times less than about 10'seconds with minimum disturbances of the working medium. In one case,the working medium was a mixture of N CO and Re which was used toproduce a lasing medium.

An important feature of volumetric ionization in accordance with theaforementioned patent application as well as the present invention is tostabilize the discharge by suppressing the arcing. Much of the arcingwhich occurs in systems in accordance with the aforementioned patentapplication occurs due to the electrode configuration and variouselectrode configurations have been tested in conjunction with thatinvention and in all cases ionization in accordance with the presentinvention creates a stabilizing effect which allows the operation inareas that heretofore would have created arcing and breakdown.

In accordance with the present invention, an electron beam is providedto produce free electrons and ionize the working medium. The electronbeam which replaces the short high-voltage pulse of the aforementionedpatent application, is, among other things, more efficient in producingionization of the working medium. For example, a 50 kv electron passingthrough air produces the order of 1000 secondary electrons along itspath before losing its energy. The effective ionization potential of agas mixture of N C0 and He is approximately 30 volts, with half theprimary electron energy loss going into ionization.

When a laser application, for example, requires very high power, thereis an advantage in working with relatively high gas pressure (such as,for example, up to one atmosphere or more) and large transversedimensions (up to 30 centimeters or more). Such conditions would requirevoltage levels in excess of 1,000,000 volts in the pulser circuit of theaforementioned patent application. The present invention eliminates thishigh voltage requirement. An electron beam ionizer in accordance withthe invention need be provided, for example, with only a voltage of theorder of kv to achieve useful ionization for such distances andpressures. Further the provision of electron beam ionization inaccordance with this invention permits continuous ionization throughsuch large volumes thereby eliminating the necessity for repetitivepulse ionization in, for example, a laser application. In addition tothe preceding, electron beam ionization can be simply and convenientlycontrolled by controlling the potential on a grid disposed in front ofthe electron emitting means. Thus, ionization level and, for example,laser output for laser applications may be simply and economicallycontrolled by controlling the grid voltage which may comprise part of alow powered, easily controlled circuit. This feature of control alongwith the ability of an electron beam to ionize in a truly continuousfashion makes apparatus in accordance with the invention highlyattractive for ionizing a working medium in any application where it isdesired or convenient to separate ionization from maintenance of adischarge.

In apparatus in accordance with the invention, at least one wall ofmeans defining a working region must transmit or provide high energyelectrons which deliver their kinetic energy to the working medium inthe form of ionization with a high efficiency. The electron beamvoltage, i.e., the energy of the electrons in the beam providing theaforementioned high energy electrons must be sufficiently high that theelectrons will enter the working region by, for example, penetration ofa diaphragm or foil disposed in a wall of the container before passingthrough and ionizing the working medium. The intensity of the electronbeam current is broadly determined by the ionization level requirementssuch that the volumetric recombination (or attachment) rate equals theproduction rate of ionization in the electron beam for a particularapplication. Increasing the intensity of the electron beam leads toincreasing the level of ionization with a corresponding highervolumetric recombination rate. The diaphragm or diaphragms through whichthe high energy electrons enter the working region need be only suchthat they transmit the necessary number of electrons and are adequatelysupported and cooled during transmission of the high energy electrons.The support requirements are such that the diaphragm must withstand thepressure differences between the working gas and the vacuum region whereprovided on the other side of the diaphragm where the high energyelectrons are created and accelerated toward the diaphragm. Typically, asuitable geometry is one where there is a high vacuum region exterior ofone or more of the walls of the cavity defining the working region.Electrons are generated in the vacuum region by any suitable method suchas, for example, plasma emission, thermionic emission, photo emission,electron bombardment and the like. Upon generation of the electrons,they are in conventional manner accelerated through a suitableelectrostatic or electromagnetic structure and caused to pass throughthe diaphragm into the working region.

Irrespective of the method of generating electrons, they may betypically coupled to the working. region through the diaphragm. Thediaphragm may be disposed over a reticulated member and in certainpulsed applications the foil temperature rise may be limited simply byits intrinsic heat capacity and may be cooled in any suitable mannersuch as by gas flow or conduction and may be comprised of Al, Be, T,, C,and

the like. Since the function of the diaphragm is to separate the workingmedium in the working region from the vacuum in the electron gun, ittypically should be capable of withstanding a pressure difference of oneatmosphere. Since the diaphragm is heated by absorbing energy fromtransmitted electrons in C. W. or numerous rapid pulse applications, itmust be cooled. However, any suitable cooling means may be used.

While a shower-hea type electron beam arrangement is shown and describedherein for irradiating a large area by a relatively low energy electronbeam of the order of 50-150 kv, it is to be understood that theinvention is not so limited and that other arrangements such as, forexample, one or a plurality of small electron guns of the type used inelectron beam welders and the like may be used where appropriate to theapplication. Further, if the use of a diaphragm is undesirable, a seriesof small holes in a plurality of plates defining a plurality of seriallydisposed chambers which are differentially pumped may be employed toprovide separation of the electron gun from the working region withoutrequiring the electrons to pass through a solid member. In this case,the electrons pass directly through one or more of a series of alignedholes in the plates and the gas in the working region will not diffuserapidly enough through the hole adjacent the electron gun tosubstantially affect the generation of electrons. Suitable voltages maybe applied to the space between plates to obtain maximum focusing of theelectrons and the pressure between plates successively decreased in thedirection of the electron gun.

Electron beam current and ionization level required in a given workingmedium are determined by the application. Thus, many N CO laserapplications require only a relatively low level of ionization and lowvolumetric beam current. Further, in this particular application, thecooling requirements of the diaphragm are modest and can besatisfactorily met by heat conduction to cooled support members.However, for MHD generator and accelerator applications, for example,higher ionization levels and higher volumetric beam currents arenecessary for practical devices. Accordingly, a greater cooling of thediaphragm will be required for this type of application than with, forexample, a laser.

The quality of the electron beam, i.e., the spread, energy anduniformity of the electron beam throughout the working medium aredetermined by the application. Thus, for many of the laser applications,the intensity of the electron beam must be substantially uniform (withvariations not exceeding about a few percent) in order to produce aworking medium with the substantially uniform ionization necessary toprovide uniform gain and optical properties in the lasing medium.

While the provision of an electron beam is preferred for the embodimentdisclosed by reason of the electron beam being a highly efficient methodof producing volumetric ionization, it is to be understood that otherapplications may require ionizing radiation in the form of photons,alpha particles, x-rays, and the like and such are included within thescope of this invention.

As may be seen from the preceding discussion, the level of ionizationthat can be obtained using high energy electrons is determined bybalancing the production rate of secondary electrons with the loss ratedue to either recombination attachment or flow. Accordingly, it isimportant to understand the limits of high energy electron currentdensity and energy to understand the relevant loss process discussedherein below.

In an embodiment actually reduced to practice, to produce laser action,an electron gun produced a stream of electrons which were directed at athin metallic foil diaphragm supported by a perforated plate with 4701/1 6 inch holes in an area 2 X 4 inches. The limiting condition on theelectron beam current was that the thin metallic foil used not be heatedto a temperature at which its structural strength was significantlyreduced, since its function is to withstand the pressure differencebetween the working region and the gun and still transmit electrons.This temperature was arbitrarily set as 200 K, the foil being aluminumhaving a thickness of cm. Other materials of other thicknesses may beused, and the foil may be cooled by a variety of means, includingconduction to cooled supports, or, for example, forced convection withgas blown across its surface in a pulse mode of operation.

In the pulse operation, we may assume that all the energy is depositedin the foil, a lower limit on the incident current density in terms ofthe incident beam energy E (volts) and the pulse length r (sec) isapproximately E I t 0.5 joule/cm 1 If E=50 kv, t=20 p. sec. I 0.5amp/cm, approximately 20 percent of the incident electrons will betransmitted with a mean energy reduction of about 10 kv and it is thesetransmitted electrons which are available for pulsed ionization of thegas. As will be seen later, the above limit represents a current densityfar in excess of that required for ionization of the gas system selectedfor lasing operation.

Consider now the processes of ionization and recombination in the gasused which is essential to proper operation of the laser:

The production rate, p, of ions in a gas per cm is dn ldt an -lp Where ais the effective recombination coefiicient and p is the production rate,it follows that in equilibrium, that is, for dn ldt 0, we have:

DE n. a]

0: T ME,

where P gas pressure, dynes cm T gas temperature, K, a effectiverecombination coefficient, cm lsec", M molecular weight of gas, m,proton mass, gm, k Boltzmans constant, erg/K.

The approximate maximum values of n for a typical electron beam andcurrent density 1 mA/cm, in a mixture of Helium, Nitrogen and CO in theproportions 3:2:1 are given immediately below, with characteristic decaytime T l/om and range R at E 50 kv, using E, 50 volts.

He:N :CO

n, R r rov-r cm Cm [Lsec To better understand the invention, the processinvolved in the creation of lasing power by the use of an electron beamand a sustainer discharge in accordance with the invention will now bediscussed.

Thermionic electrons from a tungsten filament were modulated by a gridwhose potential could be varied with respect to the filament and theelectrons were accelerated through a potential V The value of V waschosen by optimizing the ionization produced in the gas. For higherenergies Aluminum foil is more transparent and more electrons aretransmitted, but the ionization density produced is lower. Accordingly,in Eq. (2) it may be shown that 8 E/8 m z C 1n E/E, where C is aconstant,

and 8 E/8 m decreases as E increases.

The optimum value of V used was approximately 50 kv and the electron gunwas maintained at a vacuum (p 0.1 micron) and separated from the laserchamber by a thin foil of aluminum of thickness 10 cm. Aluminum waschosen simply because of its ready availability. The laser chamber maybe at any pressure from below 1p. up to about one atmosphere or more.

After passing through the foil, the electron beam entered the workingregion through a relatively wide mesh grid of stainless steel. This gridconstituted a cathode and a gold plated disk constituted an anode,between which a sustainer voltage V l0kv) was applied. The grid wasprovided to prevent damage to the foil, and the gold plate on the anodeserved to reflect a proportion of the incident primary electrons, thusincreasing the ionization of the gas. The filaments in the electron gunwere maintained at 50 kv with respect to the foil (which was at or nearground potential) by a 5 micro farad capacitor which supplied the pulsedelectron beam current. The filaments were pulsed negative 500 V withrespect to the grid. Many other schemes for projecting a beam ofelectrons into a gas are possible such as photoelectric, field emission,electron bombardment and ion bombardment.

The sustainer current was supplied by a 250 p. F capacitor at voltagesup to about 10 kv. Either the anode or the cathode can be grounded. Thevelocity of gas comprising the working medium which flowed through thelaser chamber normal to the electron beam can be varied up to aboutMach 1. In preliminary tests velocities of about one meter per secondwere used in order to ensure that the gas was uncontaminated throughleaks.

The existence of uniformity of the electron beam in the working regionand the low intensity variations of the electron beam was corroboratedby replacing the anode wall with a lucite wall coated with sodiumsalicylate, a substance that is flourescent when excited by high energyelectrons.

Two mirrors in the laser chamber, whose axis was normal to both the gasflow and the electron beam, were positioned vertically in the apparatus.One mirror was copper and concave and the otherone was IRtran 98 percentreflecting at a wavelength of l0.6p.. The mirrors were spaced 18 cmapart and were supported in a tube whose orientation could be adjustedby means of screws. The mirrors were aligned using standard techniquesand the output from laser action between the mirrors passed through a10.6 1. filter into a germanium crystal infrared detector, the output ofwhich was fed into an oscilloscope triggered by the electron beamcurrent. The sustainer current was measured as well as the infrareddetector signal and the infrared detector was calibrated with athermocouple calorimeter. It was found that laser gain became sufficientto begin lasing action only some time after the electron beam pulsereaches its maximum. This time lag represents the time required toachieve a population inversion by pumping the CO molecules into theirupper state and is sensitive to the temperature dependence of theelectron pumping rate. Increasing the sustainer voltage, and, therefore,the electron temperature in the lasing medium decreased this time lag.

A useful level of ionization was achieved with a pulse of p. sec.duration. After the pulse, the gases ionized by the pulse recombine andit is in this recombining debris stage of the cycle that laser actionoccurs. In another experiment laser action was accomplished with anessentially C. W. (i.e., ionization compared to flow and cooling times)E-beam. Thus, the electron beam pulse length may be varied from infiniteto continuous to very slow thereby creating either a truly cw laser, aneffectively cw laser, or a pulsed laser. For high power operation, theworking medium may be provided in the form of pulses and the E-beam andsustainer circuits actuated substantially between pulses. Such operationpermits substantial heat removal while still providing a substantiallyhomogeneous medium in the working region.

As discussed in the aforementioned patent application, various gases andgas mixtures may be employed to support laser action although a 3:2:1ratio of He:N CO is discussed herein, any gas or combination of gasessuch as CO, E 0, S0 HCN, NO, H Ar, N0 N 0, HF and the like may behandled in the manner discussed hereinabove and other gases may be addedif required or desired.

The present invention is applicable to substantially any useful lasergas mixture, the principal advantage of the invention being that it isapplicable to suitable gas mixtures at high pressures, producing acontrollable volumetrically scalable gas discharge over a wide range 16of operating conditions and electrode configurations. The presentinvention permits the production of a stable and controlled dischargewhen the gas mixture constituents and electron temperature T, areselected so that the rate of one or more of the variety of availablerecombination process (atom recombination, molecular recombination,attachment, etc.) exceeds the rate of ionization. When this isestablished, the discharge will not be self-sustaining, i.e., it willnot run without the ionizing means being actuated and it is this featurethat permits the ionizing means to control the dischargecharacteristics. If (T.,),,,,, is defined as the condition for aspecific gas mixture wherein ionization equals recombination, viablelaser apparatus will be provided if an inversion is produced byelectronic excitation (and/or appropriate gas kinetic de-activation ofstates related to the laser transition) at some electron temperature ortemperatures T such that T (T A specific example is the N -CO lasermixture. lonizations become significant when T is of the order of 1.5 evor higher. However, a net preferential excitation (producing aninversion) can be made to occur for electron temperatures of less than1.5 ev in both N and C0 The prior art teaches a large number of atomsand molecules that can be excited electronically by a discharge. Anylasing species which may be inverted by direct electronic excitation orby excitation via an auxiliary species (as in the N CO system) at T (Tmay be expected to be susceptible to the ionizersustainer concept andespecially the electron beam ionizer-sustainer concept of the presentinvention.

Further the present ionizer-sustainer concept may be expected to beapplicable to use of a gas mixture containing a gas which has a high netattachment rate (producing an effective recombination) at high electrontemperatures which occur, for example, in 0 for values of T up to about3 ev. Use of such a gas mixture may be expected to permit operation athigher than usual electron temperatures wherein significant ionizationof one of the lasing mixture constituents occurs. This may make lasingtransitions acceptable which are not otherwise stably available (C.W.N

TABLE I Output wavelength 10.611. Output coupling 1% Peak pulsed outputpower 3 watts EB pulse width ==lO0 usec Sustainer pulse width s 800 u.see Laser pulse width s 600 u. see Gas 16% C0,, 34% N 50% He Gaspressure 30 Torr input velocity 1 mlsec. quasistatic Laser cavity sizeDiameter: 2.54 cm Broadly, as may now be seen, the electron beam createsa desired electron density uniformly using only a small amount of energywhile the sustainer discharge provides a voltage to give these electronsa desired temperature sufficiently high for laser action for example,but not high enough to generate any appreciable increase in electrondensity. The sustainer discharge deposits the dominant amount of energyin the gas directly where it is desired. In the case of an N --C laser,the energy is put into the upper laser state of CO and into Nitrogenvibration, the optimum electron temperature assuring optimum laserefficiency. Upon creation by the electron beam of a uniform electron-ioncloud, the cloud stays uniform during the time of the electric fieldprovided by the sustainer voltage as long as the sustainer voltage doesnot result in a rapid creation of electrons. If the level of thesustainer voltage or field is raised to the point where it too producesa rapid ionization, then discharge non-uniformities may be created.However, provision of a sustainer field selected to create negligibleelectrons results in maintenance of stable, uniform and controlleddischarge for several flow times through the working region.

As will now be apparent, the present invention permits the provision ina flowing gas laser of a spatially uniform discharge at the optimumelectron temperature required for efficient laser operation at arbitrarypressure levels and physical sizes. While the invention is not solimited, this may be accomplished by utilization of the aforementionedtwo-step process comprising preferably, first an electron beam whichcreates in the gas a non-spoking predetermined spatial distribution(preferably uniform) electron density or ionization which wouldordinarily, if left on its own, disappear by volumetric processes and/orflowing out of the channel and be incapable of producing efficient highpower laser action. However, the second step or sustainer discharge isprovided which gives the electrons produced by the first step thenecessary electron temperature for preferably optimum laser (or other)excitation, with no significant increase in electron density.

It is to be understood that the invention is not limited to theapparatus shown and described and that, for example, other methods of anapparatus for creating the initial electron density can be used such asultraviolet radiation, electrical discharge, protons and the likeprovided by electron beam means for introducing one or more electronbeams to produce ionization of the gaseous medium as and for thepurposes set forth hereinabove. Irrespective of whether the electronsare generated in the above described manner or any other suitablemanner, they must be heated to the correct electron temperature by theE/N applied by the sustainer discharge.

Reference has previously been made to the fact that the presentinvention may be used to produce or facilitate carrying out chemicalprocesses such as, for example, the generation of ozone.

Heretofore, for industrial applications ozone has been principallyproduced by the use of the well-known Townsend or silent discharge.Recently a second process based on the use of a corona or high pressureglow discharge has begun to be used in commercial applications.

The Townsend discharge process is characterized by two significantoperating characteristics that are at least somewhat interdependent therequirement of a low current density in the discharge and a low overallenergy efficiency. The production of ozone requires high levels ofactivation energy even with low conversion rates of the order of onemole percent of the working medium; hence cooling is essential if anundesirable change in chemical kinetics due to temperature rise is to beprevented. The aforementioned low current density and low overall energyefficiency characteristic of the Townsend process has not only resultedin high production costs but has severely limited the application ofprocesses incorporating the Townsend discharge.

Details of the formation of ozone by a Townsend discharge are wellknown.Thus, while high positive column energy efficiencies have beenconsistently measured, a low overall efficiency results because of thesevere potential drop at the electrodes. While the dielectric layer usedto stabilize the discharge is principally responsible for theaforementioned electrode drop, without stabilization provided by thisdielectric layer, the Townsend discharge does not functionsatisfactorily as an industrial process.

The glow discharge process is not subject to the two basic deficienciesof the Townsend discharge of low current density and high cathode drop.In the glow discharge process the current density is about 2-3 orders ofmagnitude higher than that of the Townsend discharge and the cathodedrop is generally less than about 1,000 volts. However, compared to theTownsend discharge, the positive column energy efficiency of the glowdischarge process is significantly lower.

Glow discharge processes are generally conducted under low pressure andwith walls cooled to liquid air temperature. On the other hand, the highpressure discharge, or corona discharge, is subject to a more limitedstability range than the low pressure glow discharge. Further, with theexception of a high frequency electrodeless discharge, electrodegeometry is generally a critical factor in achieving stabilization of acorona discharge.

Due to the necessity of stabilization by a dielectric layer or specificelectrode geometry, the abovedescribed processes are essentially surfaceprocesses. In such processes the kinetics in the active volume of theworking medium is not homogeneous and optimum or near optimum conditionscannot be uniformly established.

The basic problem in the above-described processes is believed to existbecause of close coupling between the emission of electrons from theelectrodes and the electric field in the active volume. Where suchcoupling exists, it is very difficult to maintain a steady and uniformdischarge without arcing.

Ozone may be more easily and efficiently produced than heretofore byutilizing, in accordance with the present invention, an independentsource of electrodes in the form of an electron beam or, alternately,repeated short electron beam pulses superimposed on a sustainingelectric field as and for the purposes hereinbefore described. As in alaser application, in this case the electric field in the active volumeis also decoupled from the requirement of the electron emmission,whereby optimum conditions for ozone formation may be provided withoutfor example, severe requirements on ballasting as required for aTownsend discharge or severe requirements on electrode geometry as inthe case of a corona discharge. The production of ozone in accordancewith the present invention is a truly volumetric process; hence forlarge scale applications, not only are scaling problems simpler thanwith prior art processes, but overall equipment size can be drasticallyreduced. Furthermore, the uniform conditions provided in the activecolumn in accordance with the present invention provides an improvementin the overall energy efficiency and minimizes heat dissipation involvedin the process as compared to that of the prior art. Accordingly, ozonemay be produced in accordance with the present invention in higherconcentrations than that heretofore available without the necessity ofcooling.

Ozone may be produced with apparatus substantially as shown anddescribed hereinabove with the exception that the mirror means definingthe optical cavity are not required. The working medium for theproduction of ozone may be air or preferably pure oxygen. Electrons aregenerated by the electron gun in the manner previously described, enterthe working region, and collide with oxygen molecules to form secondaryelectrons and ions. The electron temperature in the working region mustbe maintained at a level which is favorable to ozone production.

in the working region, electrons are generated through ionization byprimary and secondary electrons and are lost by attachment to themolecules of oxygen. Both the ionization rate of secondary electrons andthe electron attachment rate to oxygen molecules are strongly influencedby the electron temperature. Since, in accordance with the invention,there is an excess of electrons due to ionization by the primaryelectrons, a spatially uniform current can be maintained with anionization rate of secondary electrons less than the attachment rate.Accordingly, the stability of the discharge process in the workingregion is substantially greater than that in conventional dischargeprocesses where net ionization by secondary electrons is essential tosustain the discharge process.

If a secondary electron temperature in the range of approximately 2-3electron volts is provided, a large percentage of the energy lost inelastic collision goes into dissociation of oxygen which is essential tothe production of ozone in high concentrations. The electron temperaturerange suitable for the production of ozone is higher by about a factorof 2 than the range necessary to produce laser action. Further, since alow ambient temperature may be easily provided in the working region,this permits the production of higher concentration of ozone with muchgreater efficiency than heretofore possible.

Another application of the present invention is to magnetohydrodynamicpower generation '(MHD). Electric power can be extracted from anelectrically conductive stream of plasma by passing the plasma through amagnetic field transverse to the direction of flow. The magnetic fieldcreates an electric field perpendicular to the magnetic field and to thedirection of flow, and suitably constructed electrodes arranged parallelto the electric field permit the kinetic energy of the plasma to becoupled out as electrical energy. For a more complete discussion ofMl-lD devices, reference is made to U. S. Pat. No. 3,264,501.

Ali

In this type of application, an electron beam in accordance with theinvention is injected into the plasma to maintain the required level ofionization independent of the electron temperature. In this manner, astable plasma discharge may be usefully produced in the plasma whereinthe ionization is volumetric and stabilized not by ambipolar diffusionof ion pairs to the walls as in a conventional discharge, but byequilibrium between ion recombination and ion production by the electronbeam. It is to be emphasized that the parameters set forth below andtheir numerical values are given only by way of illustration.

Requirements on Recombination Coefficient MHD Generator Gas: Helium FlowVelocity l .5 X l0 m/sec Flow time: -10 sec Energy Extracted fromplasma: ===O.2 eV/particle lonization Level: n, l0 electrons/cm GasDensity: 3 X 10" cm" Effective energy per ionization 50 eV Energyrequired per particle to ionize: Assuming I00 ionizations per flow time,energy required to maintain ionization Ratio:

This must be H 100 of the flow time, giving a requirement for therecombination coefficient: a2 l0ln, 10 cmlsec Recent experimentalresults (Berlande et al, Phys Rev A1, 887, 1970) indicate that in thepreferred working region, n,=10, electron temperature T, z 3 X 10 l(,gas temperature Tg==l300l(, the upper limit on the effectiverecombination coefficient is a 10 em /sec.

The electron beam current density required to maintain an equilibriumionization level n, 10 cm is ob tained by equating production rate andrecombination rate. Thus: om, IE/eE R(E), approximately where aeffective recombination coefficient I EB current density, amp cm Eelectron energy, volts R(E) range of electrons of energy E, cm

E, 50 eV per ion pair e 5/3 X 10' coulombs For E 100 keV, and densityN,, 2.6 X 10 cm', then R(E) z cm in helium 7.5 X 10" amps/cm when a l0"cm lsec n An electron beam as set forth above is easily produced as andfor the purposes previously described and may include for example usinga jet of helium to cool the foil or diaphragm, the beam of electronsbeing injected into the MHD channel at a suitably chosen angle relativeto the direction of the applied magnetic field.

manna The various features and advantages of the invention are thoughtto be clear from the foregoing description. Various other features andadvantages not specifically enumerated will undoubtedly occur to thoseversed in the art, as likewise will many variations and modifications ofthe preferred embodiment illustrated, all of which may be achievedwithout departing from the spirit and scope of the invention as definedby the following claims:

We claim:

1. In the method of producing a spatially uniform controlled dischargesubstantially throughout a gaseous working medium in a working region,the steps comprising:

a. providing a gaseous working medium at a pressure in a working regiondisposed in a cavity having imperforate walls for confining the gaseousworking medium that upon the production of secondary electrons in saidmedium said medium has ambipolar and thermal diffusion rates incapableof damping local increases in secondary electron density in said medium;

b. generating ionizing radiation externally of said cavity; introducingsaid ionizing radiation into said cavity through one of said walls toproduce substantially throughout said working region a substantiallyspatially uniform predetermined density of secondary electrons in saidmedium by ionizing said medium, said one wall being impervious to gasesand pervious to said ionizing radiation; and

d. providing a sustainer field for providing substantially uniformlythroughout said working region a predetermined electron temperatureeffective to increase the average energy of said secondary electronswithout substantially increasing said predetermined electron density byself-regenerative ionization, said electron temperature producing saidcontrolled discharge substantially uniformly throughout said workingregion at a predetermined level.

2. The method as defined in claim 1 wherein said electron temperature iscontrolled at least in part by flowing said medium through said cavity.

3. The method as defined in claim 2 wherein said density and temperatureare maintained at said values less than that which will produceuncontrolled arcing for times less than the characteristic time of saiddischarge.

4. The method as defined in claim 1 wherein the density of saidsecondary electrons is controlled at least in part by flowing saidmedium through said cavity.

5. The method as defined in claim 1 wherein said working medium ispassed through said cavity.

6. In the method of producing a spatially uniform controlled dischargesubstantially throughout a gaseous working medium in a working region,the steps comprising:

a. passing a gaseous working medium at a pressure through a workingregion disposed in a cavity having imperforate walls for confining thegaseous working medium that upon the production of secondary electronsin said medium said medium has ambipolar and thermal diffusion ratesincapable of damping local increases in secondary electron density insaid medium;

b. generating ionizing radiation externally of said cavity;

c. introducing said ionizing radiation into said cavity through one ofsaid walls to produce substantially throughout said working region asubstantially spatially uniform predetermined density of secondaryelectrons in said medium by ionizing said medium, said one wall beingimpervious to gases and pervious to said ionizing radiation;

d. providing a sustainer field for providing substantially uniformlythroughout said working region a predetermined electron temperatureeffective to increase the average energy of said secondary electronswithout substantially increasing said predetermined electron density byself-regenerative ionization; and

. providing a pressure and velocity of said medium in said workingregion to produce said controlled discharge substantially uniformlythroughout said working region at a predetermined level.

7. In apparatus for producing a controlled discharge for providingmolecular excitation of a gaseous working medium, the combinationcomprising:

a. means defining a cavity having a working region disposed therein,said cavity having imperforate walls for confining a gaseous workingmedium and defining a predetermined cross section and volume;

. a working medium in said cavity and working region at a pressure thatupon the production of free electrons in said medium at said pressuresaid medium has ambipolar and thermal diffusion rates incapable ofdamping local increases in electron density in said medium;

. first means for generating ionizing radiation externally of saidcavity;

. second means for introducing said ionizing radiation into said cavitythrough one of said walls and producing substantially throughout saidworking region a substantially uniform predetermined density ofsecondary electrons in said medium by ionizing said medium, said onewall being impervious to gases and pervious to said ionizing radiation;and

. third means for providing a sustainer field for providingsubstantially throughout said working region a predetermined electrontemperature of said secondary electrons effective to increase theaverage energy of said secondary electrons without substantiallyincreasing said predetermined electron density by self-regenerativeionization, said electron temperature producing said controlleddischarge substantially uniformly throughout said working region at apredetermined level.

8. The combination as defined in claim 7 wherein said cavity includesgas inlet and gas outlet means, and additionally including fourth meanscoupled to said gas inlet for flowing said medium through said cavity.

9. The combination as defined in claim 8 wherein said fourth meansincludes further means for providing a predetermined pressure andvelocity of said medium in said working region.

10. The combination as defined in claim 8 and addi-l;

tionally including diaphragm means separating said first means and saidcavity, said ionizing radiation being introduced into said mediumthrough said diaphragm intermediate said gas inlet and said gas outletand normal to the direction of flow of said medium through said cavity.

11. The combination as defined in claim wherein said third meansincludes electrode means for providing a sustainer electric field insaid cavity normal to the direction of flow of said medium, saidelectrode means comprising a first electrode adjacent the wall throughwhich said radiation is introduced and through which said radiationpasses, and a second electrode oppositely disposed to said firstelectrode adjacent the opposite wall of said cavity.

12. The combination as defined in claim 11 wherein said second meansincludes a perforate plate member and a thin diaphragm covering andcarried by said plate member, said diaphragm being disposed between saidplate member and said medium.

13. The combination as defined in claim 7 wherein:

a. said medium has an upper and lower laser state;

b. said first and second means provides a density of secondary electronsin said medium sufficient to support a population inversion; and

c. said third means increases the average energy of said secondaryelectrons to a level to produce a population inversion in said medium insaid cavity.

14. The combination as defined in claim 13 and additionally including:

a. means for passing said medium through said cavity in the form ofpulses; and

b. means for actuating said first, second and third means intermediatesaid pulses to produce said population inversion intermediate saidpulses.

15. The combination as defined in claim 13 wherein said medium iscontinuously passed through said cavity.

16. In the method of light generation by stimulated emission ofradiation substantially throughout a gaseous active medium in a workingregion, the steps comprising:

a. providing a gaseous active medium at a pressure in a working regiondisposed in a cavity having imperforate walls for confining the gaseousworking medium that upon the production of secondary electrons in saidmedium said medium has ambipolar and thermal diffusion rates incapableof damping local increases in secondary electron density in said medium,said medium having an upper and lower laser state;

b. generating externally of said cavity a broad area electron beamhaving a cross sectional area conforming substantially to that of saidworking region;

c. introducing said electron beam into said cavity through one of saidwalls to produce substantially throughout said working region asubstantially spa tially uniform predetermined density of secondaryelectrons in said medium having an average energy insufficient toproduce a population inversion in said medium, said one wall beingimpervious to gases and pervious to said electron beam; and

d. providing a sustainer field for providing substantially uniformlythroughout said working region a predetermined electron temperatureeffective to increase the average energy of said secondary electronswithout substantially increasing said predetermined electron density byself-regenerative ionization, said electron temperature producing anaverage energy level sufficient to support a population inversion insaid medium.

17. The method as defined in claim 16 wherein said medium is at leastsequentially passed through said cavity at a pressure and velocity toproduce substantially uniformly throughout said working region apopulation inversion in said medium.

18. The method as defined in claim 17 wherein the electron temperatureis controlled by providing a sustainer electric field in said medium andsaid pressure and velocity are provided to produce substantially maximumpopulation inversion in said working region.

19. The method as defined in claim 16 wherein said population inversionis serially provided in the form of pulses and the energy added to themedium by the in- W troduction of said free electrons is less than theenergy added to the medium by said sustainer field.

20. In high powered laser apparatus the combination comprising:

a. gas supply means for producing a flow of a gaseous medium having apredetermined velocity and pressure and an upper and lower laser state;

. means defining a cavity including a working region for receiving saidmedium from said gas supply means and through which said flow passes;

c. first means for generating externally of said cavity a broad areaelectron beam having a cross sectional area conforming substantially tothat of said working region, said means defining said cavity includingwalls for confining said medium, one of said walls including a diaphragmimpervious to said medium and pervious to said electron beam;

d. second means for introducing said electron beam into said cavitythrough said diaphragm forming a part of said one of said walls of saidcavity and produce a substantially uniform spatial distribution ofsecondary electrons in said medium in said working region by ionizingsaid medium, said secondary electrons having an average energyinsufficient to produce a population inversion in said medium; and

e. third means for providing a sustainer field for controlling theelectron temperature of said secondary electrons in said medium tosubstantially uniformly throughout said working region increase theiraverage energy without substantially increasing the density thereof byself-regenerative ionization at said velocity and pressure and produce apopulation inversion in said medium in said working region.

21. The combination as defined in claim 20 wherein said third meansincludes means for generating a sustainer electric field in said cavity.

22. The combination as defined in claim 21 wherein said means forgenerating said sustainer electric field includes first and secondelectrode means in said cavity.

23. The combination as defined in claim 22 wherein said second electrodemeans is comprised of a perforate member and disposed in spacedrelationship over said diaphragm.

24. The combination as defined in claim 23 wherein said working regionincludes means for passing a light beam through said working region.

said first means adds energy to said medium in an amount that is lessthan that of said third means.

29. The combination as defined in claim 20 and additionally including:

a. A perforate plate member carried by said means defining said cavityand covered by said diaphragm, said diaphragm being disposed over saidplate member and between it and said medium in said cavity.

UNETEE STATES PATENT @FFECEE I CERHFEQATE @F CURREQTEQN Patent No. 9Dated November 14, 1972 Jack D. Daugherfiy, Dial" id H.Douglas-Hamilton, Inventor(s) Richard M. Patrick and E an R. Pugh It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below? E" u i -1 InAbstract, line 14, for "lasting read--lasing--; Column 13, line 51, forken/anew read-- 8 E/a m Column 13, line 57, for "dn /dt (In p" read--dn/d1i= 1n p--; Column 14, line 39, for 5E/5m read-- 8 E/B rn Column 14,line 40, for 6E/i5rn read-- B E/B m andColurnn 15, line 58, for "HezN COread--He:N

Signed and sealed this 29th day of Ma) 1973'.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesfting Officer Commissionerof Patents

1. In the method of producing a spatially uniform controlled discharge substantially throughout a gaseous working medium in a working region, the steps comprising: a. providing a gaseous working medium at a pressure in a working region disposed in a cavity having imperforate walls for confining the gaseous working medium that upon the production of secondary electrons in said medium said medium has ambipolar and thermal diffusion rates incapable of damping local increases in secondary electron density in said medium; b. generating ionizing radiation externally of said cavity; c. introducing said ionizing radiation into said cavity through one of said walls to produce substantially throughout said working region a substantially spatially uniform predetermined density of secondary electrons in said medium by ionizing said medium, said one wall being impervious to gases and pervious to said ionizing radiation; and d. providing a sustainer field for providing substantially uniformly throughout said working region a predetermined electron temperature effective to increase the average energy of said secondary electrons without substantially increasing said predetermined electron density by self-regenerative ionization, said electron temperature producing said controlled discharge substantially uniformly throughout said woRking region at a predetermined level.
 2. The method as defined in claim 1 wherein said electron temperature is controlled at least in part by flowing said medium through said cavity.
 3. The method as defined in claim 2 wherein said density and temperature are maintained at said values less than that which will produce uncontrolled arcing for times less than the characteristic time of said discharge.
 4. The method as defined in claim 1 wherein the density of said secondary electrons is controlled at least in part by flowing said medium through said cavity.
 5. The method as defined in claim 1 wherein said working medium is passed through said cavity.
 6. In the method of producing a spatially uniform controlled discharge substantially throughout a gaseous working medium in a working region, the steps comprising: a. passing a gaseous working medium at a pressure through a working region disposed in a cavity having imperforate walls for confining the gaseous working medium that upon the production of secondary electrons in said medium said medium has ambipolar and thermal diffusion rates incapable of damping local increases in secondary electron density in said medium; b. generating ionizing radiation externally of said cavity; c. introducing said ionizing radiation into said cavity through one of said walls to produce substantially throughout said working region a substantially spatially uniform predetermined density of secondary electrons in said medium by ionizing said medium, said one wall being impervious to gases and pervious to said ionizing radiation; d. providing a sustainer field for providing substantially uniformly throughout said working region a predetermined electron temperature effective to increase the average energy of said secondary electrons without substantially increasing said predetermined electron density by self-regenerative ionization; and e. providing a pressure and velocity of said medium in said working region to produce said controlled discharge substantially uniformly throughout said working region at a predetermined level.
 7. In apparatus for producing a controlled discharge for providing molecular excitation of a gaseous working medium, the combination comprising: a. means defining a cavity having a working region disposed therein, said cavity having imperforate walls for confining a gaseous working medium and defining a predetermined cross section and volume; b. a working medium in said cavity and working region at a pressure that upon the production of free electrons in said medium at said pressure said medium has ambipolar and thermal diffusion rates incapable of damping local increases in electron density in said medium; c. first means for generating ionizing radiation externally of said cavity; d. second means for introducing said ionizing radiation into said cavity through one of said walls and producing substantially throughout said working region a substantially uniform predetermined density of secondary electrons in said medium by ionizing said medium, said one wall being impervious to gases and pervious to said ionizing radiation; and e. third means for providing a sustainer field for providing substantially throughout said working region a predetermined electron temperature of said secondary electrons effective to increase the average energy of said secondary electrons without substantially increasing said predetermined electron density by self-regenerative ionization, said electron temperature producing said controlled discharge substantially uniformly throughout said working region at a predetermined level.
 8. The combination as defined in claim 7 wherein said cavity includes gas inlet and gas outlet means, and additionally including fourth means coupled to said gas inlet for flowing said medium through said cavity.
 9. The combination as defined in claim 8 wherein said fourth means includes further means for providing a predetermined pressure and velocitY of said medium in said working region.
 10. The combination as defined in claim 8 and additionally including diaphragm means separating said first means and said cavity, said ionizing radiation being introduced into said medium through said diaphragm intermediate said gas inlet and said gas outlet and normal to the direction of flow of said medium through said cavity.
 11. The combination as defined in claim 10 wherein said third means includes electrode means for providing a sustainer electric field in said cavity normal to the direction of flow of said medium, said electrode means comprising a first electrode adjacent the wall through which said radiation is introduced and through which said radiation passes, and a second electrode oppositely disposed to said first electrode adjacent the opposite wall of said cavity.
 12. The combination as defined in claim 11 wherein said second means includes a perforate plate member and a thin diaphragm covering and carried by said plate member, said diaphragm being disposed between said plate member and said medium.
 13. The combination as defined in claim 7 wherein: a. said medium has an upper and lower laser state; b. said first and second means provides a density of secondary electrons in said medium sufficient to support a population inversion; and c. said third means increases the average energy of said secondary electrons to a level to produce a population inversion in said medium in said cavity.
 14. The combination as defined in claim 13 and additionally including: a. means for passing said medium through said cavity in the form of pulses; and b. means for actuating said first, second and third means intermediate said pulses to produce said population inversion intermediate said pulses.
 15. The combination as defined in claim 13 wherein said medium is continuously passed through said cavity.
 16. In the method of light generation by stimulated emission of radiation substantially throughout a gaseous active medium in a working region, the steps comprising: a. providing a gaseous active medium at a pressure in a working region disposed in a cavity having imperforate walls for confining the gaseous working medium that upon the production of secondary electrons in said medium said medium has ambipolar and thermal diffusion rates incapable of damping local increases in secondary electron density in said medium, said medium having an upper and lower laser state; b. generating externally of said cavity a broad area electron beam having a cross sectional area conforming substantially to that of said working region; c. introducing said electron beam into said cavity through one of said walls to produce substantially throughout said working region a substantially spatially uniform predetermined density of secondary electrons in said medium having an average energy insufficient to produce a population inversion in said medium, said one wall being impervious to gases and pervious to said electron beam; and d. providing a sustainer field for providing substantially uniformly throughout said working region a predetermined electron temperature effective to increase the average energy of said secondary electrons without substantially increasing said predetermined electron density by self-regenerative ionization, said electron temperature producing an average energy level sufficient to support a population inversion in said medium.
 17. The method as defined in claim 16 wherein said medium is at least sequentially passed through said cavity at a pressure and velocity to produce substantially uniformly throughout said working region a population inversion in said medium.
 18. The method as defined in claim 17 wherein the electron temperature is controlled by providing a sustainer electric field in said medium and said pressure and velocity are provided to produce substantially maximum population inversion in said working region.
 19. The method as defined in claim 16 wherein saiD population inversion is serially provided in the form of pulses and the energy added to the medium by the introduction of said free electrons is less than the energy added to the medium by said sustainer field.
 20. In high powered laser apparatus the combination comprising: a. gas supply means for producing a flow of a gaseous medium having a predetermined velocity and pressure and an upper and lower laser state; b. means defining a cavity including a working region for receiving said medium from said gas supply means and through which said flow passes; c. first means for generating externally of said cavity a broad area electron beam having a cross sectional area conforming substantially to that of said working region, said means defining said cavity including walls for confining said medium, one of said walls including a diaphragm impervious to said medium and pervious to said electron beam; d. second means for introducing said electron beam into said cavity through said diaphragm forming a part of said one of said walls of said cavity and produce a substantially uniform spatial distribution of secondary electrons in said medium in said working region by ionizing said medium, said secondary electrons having an average energy insufficient to produce a population inversion in said medium; and e. third means for providing a sustainer field for controlling the electron temperature of said secondary electrons in said medium to substantially uniformly throughout said working region increase their average energy without substantially increasing the density thereof by self-regenerative ionization at said velocity and pressure and produce a population inversion in said medium in said working region.
 21. The combination as defined in claim 20 wherein said third means includes means for generating a sustainer electric field in said cavity.
 22. The combination as defined in claim 21 wherein said means for generating said sustainer electric field includes first and second electrode means in said cavity.
 23. The combination as defined in claim 22 wherein said second electrode means is comprised of a perforate member and disposed in spaced relationship over said diaphragm.
 24. The combination as defined in claim 23 wherein said working region includes means for passing a light beam through said working region.
 25. The combination as defined in claim 23 wherein said cavity includes means defining an optical cavity in said working region.
 26. The combination as defined in claim 25 and additionally including control means for actuating said second and third means in the pulsed mode.
 27. The combination as defined in claim 25 and additionally including control means for actuating said second and third means in the continuous mode.
 28. The combination as defined in claim 20 wherein said first means adds energy to said medium in an amount that is less than that of said third means.
 29. The combination as defined in claim 20 and additionally including: a. A perforate plate member carried by said means defining said cavity and covered by said diaphragm, said diaphragm being disposed over said plate member and between it and said medium in said cavity. 